12 crew members missing after a Chevron-contracted tug vessel has sank on Sunday due to rough seas off the coast of Nigeria, told the owner of the sunken vessel on Tuesday.The Jascon-4 tug vessel capsized early on Sunday due to "rough ocean swells" while the ship was "performing towing operations" at a mooring point around 23 nautical miles off oil-producing Delta state, Chevron's Nigeria unit commented on Sunday."Unfortunately all 12 crew members of the tug vessel are still missing," a spokeswoman for the vessel's owner West African Ventures commented."Rescue actions involving sea divers are still ongoing and a full investigation into the accident is taking place," she told. West African Ventures is owned by Sea Trucks Group.Chevron Nigeria declined to give any update on the accident.Chevron's operates offshore and onshore joint ventures with the Nigerian National Petroleum Corp and spends around 3 billion dollars a year on its Nigerian operations, the company commented.